User terminals are used to retrieve information from a network server, to connect via is the network to another user terminal, and for many further purposes that require a network connection. The network capabilities of user terminals depend on the supported network access technologies. A mobile telephone, for example, typically supports a cellular access technology according to the GSM (Global System for Mobile communication) standard, a WCDMA (Wideband Code Division Multiple Access) standard or a similar cellular standard. For a personal computer, on the other hand, support of a WLAN (Wireless Local Area Network) access technology is widespread.
In recent years, the number of WLAN hot spots and the GSM/WCDMA coverage has steadily increased, and in many geographical places two or even more different access technologies are available at the same time. As a result of the increased availability of network accesses, modern user terminals often support two or even more different network access technologies. Support of more than one network access technology is generally advantageous because—provided that multiple network accesses are actually available at the current location of the user terminal—the user terminal may dynamically select a specific network access. While data communication, for example, is typically faster via a WLAN access, a GSM/WCDMA access is generally better suited for voice communication. There are, of course, many further considerations why to select a particular network access, including cost arguments and subscription agreements.
For user terminals that support more than one access technology, service continuity is an important aspect. The paradigm of service continuity implies that a transfer from a first network access to a second network access can be performed without interrupting a service currently provided to the user terminal.
A well known service continuity concept is Voice Call Continuity (VCC). VCC is a mobility scheme on a call control level that has been defined by the Third Generation Partnership Project (3GPP) in various technical specifications, including Technical Specification Group Services and System Aspects; Voice Call Continuity between CS and IMS Study (Release 7), 3GPP TR 23.806, Version 1.7.0 and Technical Specification Group Services and System Aspects; Voice Call Continuity between CS and IMS; Stage 2 (Release 7), 3GPP TS 23.206, Version 0.3.0.
With VCC, a voice call can be transferred for example between a GSM or WCDMAbased circuit-switched (CS) access domain on the one hand and a WLAN or WCDMAbased packet-switched (PS) access domain on the other hand without disconnecting the call. In this regard, FIG. 1 illustrates an exemplary access domain transfer scenario for a voice call in accordance with the 3GPP VCC scheme, for a mobile user terminal 10 supporting GSM/WCDMA as well as WLAN access technologies.
In the scenario of FIG. 1, one has to distinguish between access domain selection on the one hand and service domain selection on the other. It is assumed here that the GSM/WCDMA network access will be performed via a CS access domain 12, while the WLAN network access will be performed via a PS access domain 14. It is further assumed that the GSM/WCDMA voice services are provided in a CS service domain 16 by a mobile network operator, while the WLAN voice services are provided in a PS service domain 18 by a Voice-over-IP (VoIP) provider or a provider of an IP Multimedia Subsystem (IMS).
Now consider the situation in which the user terminal 10 is roaming during an ongoing voice call between a first area with GSM/WCDMA coverage and a second area with both GSM/WCDMA and WLAN coverage. When the user terminal 10 is roaming from the first area into the second area, and if the WLAN access is preferred, a VCC application residing on the user terminal 10 will initiate an access and service domain transfer from CS-GSM/WCDMA to IMS/VoIP over WLAN. Should the user terminal 10 at a later point in time roam back into a portion of the area without WLAN coverage, the opposite transfer will take place, i.e. from IMS/VoIP over WLAN to CSGSM/WCDMA. The scenario shown in FIG. 1 requires that the user terminal 10 supports a dual-radio mode to have a CS voice call and a VoIP/IMS Multimedia Telephony (MMTeI) session in parallel (at least during the access transfer phase).
According to the VCC specifications, the transfer from a first access domain to a second access domain is triggered by the user terminal. In addition to the preference-based transfer discussed above with reference to FIG. 1, the VCC specifications also describe an access domain transfer that is performed depending on radio conditions. This means that if the user terminal determines that coverage of one access domain starts to fade away or appear, the VCC application running on the user terminal may decide about a transfer to another access domain with better radio conditions.
While a transfer of the access domain triggered by user preferences and radio conditions is an important feature, it would be desirable to allow the network side to control or at least influence the access domain switching in certain cases. Accordingly, there is a need for an access domain switching technique that involves the network side.